Skin or plating of some kind?

[Micah]

I found this in some glacial till in a creek bed here in southeast Nebraska and I have absolutely no idea what it is. It kinda looks like  alligator or crocodile skin, but it could be fossilized mud for all I know (seems a bit too regular and layered for that though). Maybe someone who knows more than me can help!

 

[abyssunder]

I think you are on the good track.
To me it looks like ancient laminated mudstone or similar, with propagation cracks. Also it could be similar to septarian. (just few of the interpretations). In the document below is nicely described and exemplified the evolution of crack patterns during the drying process.

 

" Cracks in thin layers are influenced by what lies beneath them. From buried craters to crocodile skin, crack patterns are found over an enormous range of length scales. Regardless of absolute size, their substrates can dramatically influence how cracks form, guiding them in some cases, or shielding regions from them in others. Here we investigate how a substrate's shape affects the appearance of cracks above it, by preparing mud cracks over sinusoidally varying surfaces. We find that as the thickness of the cracking layer increases, the observed crack patterns change from wavy to ladderlike to isotropic. Two order parameters are introduced to measure the relative alignment of these crack networks, and, along with Fourier methods, are used to characterise the transitions between crack pattern types. Finally, we explain these results with a model, based on the Griffith criteria of fracture, that identifies the conditions for which straight or wavy cracks will be seen, and predicts how well-ordered the cracks will be. Our metrics and results can be applied to any situation where connected networks of cracks are expected, or found. "

 

P. Nandakishore, L. Goehring. 2016. Crack patterns over uneven substrates. Soft Matter 12 (8), 2253-2263

[Darktooth]

@abyssunder How do you even find this stuff?

[Micah]

@abyssunder Super interesting article (a bit above my head in terms of mathematic descriptions of how and why cracks form, but still informative). After looking at lots of images of mudstone and septarian I'm inclined to lean more towards septarian. I couldn't figure out though if septarian forms layers like those in the piece I found and if the "cracks" can be raised as in piece.

[abyssunder]

Usually the septarian concretions/nodules are spherical in shape. You specimen looks to be flat and layered. Unfortunately, we don't know the geological settings, the mineral/chemical composition of the specimen in question, nor how the cracks were formed. One could say they are syneresis cracks , not desiccation cracks .

 

" Mudcracks and Syneresis Cracks

 

Mudcracks in modern sediment are downward-tapering, V-shaped fractures that display a crudely polygonal pattern in plan view. The area between the cracks is commonly curved upward into a concave shape. Mudcracks form in both siliciclastic and carbonate mud owing to desiccation. Subsequent sedimentation over a cracked surface fills the cracks. In ancient sedimentary rocks, mudcracks are commonly preserved on the tops of bedding surfaces as positive-relief fillings of the original cracks (Fig. 5.41).
The mudcrack polygons range in diameter from a few centimeters to a few meters. The cracks themselves commonly range in width up to a few centimeters and in depth to a few tens of centimeters, but cracks up to a few meters in depth have been reported.
The presence of undoubted mudcracks indicates intermittent subaerial exposure; however, mudcracks can be confused with syneresis cracks (below), which form under water. Mudcracks occur in estuarine, lagoonal, tidal-flat, river floodplain, playa-lake, and other environments where muddy sediment is intermittently exposed and allowed to dry. They may be associated with raindrop or hailstone imprints, bubble imprints and foam impressions, flat-topped ripple marks, and vertebrate tracks (Plummer and Gostin, 1981).
In contrast to the continuous, polygonal network of mudcracks that occurs on bedding surfaces, syneresis cracks tend to be discontinuous and vary in shape from polygonal to spindle-shaped or sinuous (Plummer and Gostin, 1981). They commonly occur in thin mudstones interbedded with sandstones as either positive-relief features on the base of the sandstones or negative-relief features on the top of the mudstones.

Syneresis cracks are subaqueous shrinkage cracks that form in clayey sediment by loss of pore water from clays that have flocculated rapidly or that have undergone shrinkage of swelling-clay mineral lattices owing to changes in salinity of surrounding water (Burst, 1965). They are known in ancient sedimentary rocks from both marine and nonmarine environments. They may be confused with mudcracks and even some trace fossils. For example, the lenticular shape of the crack fill in plan view may resemble burrow traces. Because some syneresis cracks do closely resemble mudcracks, it is important in trying to differentiate them to look for features associated with mudcracks that indicate subaerial exposure-features such as raindrop imprints and vertebrate tracks. " - as stated here